A laser is a device that produces light through the stimulated emission of photons from an active gain medium. Such a device contains an optical cavity or resonator, wherein light circulates between a highly reflective mirror and partially transmissive mirror, which is responsible for the emission of the output laser beam. With each round trip, the circulating light passes through the gain medium that amplifies the circulating light and compensates for resonator losses. In order to amplify the circulating light, the gain medium is “pumped” with an external source of energy such as light (known as optical pumping) or electrical current (known as electrical pumping).
Solid state lasers, wherein the gain medium comprises a ion-doped crystalline solid host (e.g., neodymium-doped ytrrium aluminum garnet or Nd:YAG), are desirable for a number of military and civilian applications. Military applications include use as rangefinders, target designators, and illuminators. Civilian applications include spectroscopy, laser surgery, and laser radars. Most of these applications require short-pulse (<=20 ns) operation, wherein the laser output varies with time, as opposed to continuous wave operation, wherein the laser output is relatively constant with time. The advantage of short-pulse operation is that relatively high amounts of energy (e.g., peak power in the gigawatt range) can be concentrated in a given place in as short a time as possible.
Q-switching is a technique for producing energetic short pulses. The term “Q” refers to the Q factor or quality factor, which represents the ratio of energy stored in a resonant cavity to the energy loss per cavity round trip. The Q-switching technique uses a device known as the “Q-switch.” The Q-switch increases the resonator losses until such time that the stored energy is sufficient to allow the generation of a high peak power optical pulse. Then, the cavity losses are suddenly reduced. In other words, a Q-switch modulates resonator losses so as to prevent or induce laser output as required. The Q-switch inhibits laser output in the low “Q” (or high-loss) state known as holdoff. When the Q-switch is switched to its high “Q” (or low-loss) mode, the laser is suddenly able to release a substantial fraction of the stored energy in a very short time, producing very high peak powers.
Q-switches comprising optical modulators based on the electro-optic effect are commonly used in short pulse lasers. Such optical modulators have crystals of a suitable material, e.g., lithium niobate (LiNbO3), whose optical properties may be varied in proportion to an applied electric field. However, one drawback of using lithium niobate, or other high surface resistivity crystals, is that they can suffer from surface charging due to temperature changes via the pyroelectric effect or via other means. If the surface charges are not neutralized, then there is loss of holdoff, leading to premature laser output known as prelase. This can adversely affect the performance of the laser and decrease the reliability of laser products containing a Q-switch.
U.S. Pat. No. 4,884,044 to Heywood et al. (reissued as RE 35,240) discloses several methods for neutralizing surface charge induced by the pyroelectric effect including corona discharge, spark gaps, alpha particle emitting radioactive sources, conductive coatings, and a conductive wiper. In a first method, electrodes near the end faces of a crystal are connected to a high voltage power supply. When the power supply is turned on, it generates a stream of ions which neutralizes the surface charge. A drawback of this method is that the power supply associated with the discharge is bulky and prone to electromagnetic interference (“EMI”). A similar drawback is observed in a second disclosed method, wherein ions are generated via a spark discharge near the crystal surfaces. Moreover, the spark discharge erodes the electrodes and causes damaging deposits on the crystal's optical surfaces.
In a third method described in the '044 Patent, a radioactive source, such as an alpha emitter (e.g., Americium 241), is placed near the crystal surfaces. The radioactive source ionizes the air, thereby neutralizing the temperature-induced surface charge. The use of a radioactive source has been the solution of choice for military lasers, but customers demand a non-radioactive method, especially because a vendor is required to set up the infrastructure that is needed to properly control and dispose of the radioactive sources.
In a fourth method described in the '044 Patent, an optically transparent, electrically conductive coating is applied to the crystal surfaces. However, in this passive method, the electrically conductive coatings are historically prone to failure due to laser induced breakdown. In a fifth method, an electrically conducting surface is used to periodically wipe the charge surface like a wiper blade. However, this is a complicated mechanism which is prone to scratch the optical surface. Moreover, these methods require physical contact with crystal surfaces.
Thus, there still remains a need for a compact, non-radioactive, non-contact ionization source that can passively neutralize the surface charge induced by the pyroelectric effect or other means. There also remains the further need for an ionization source that does not interfere with the normal Q-switched operation of a laser beam.